This invention relates to magnetorheological fluids.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are the electrorheological (ER) fluids which exhibit a like ability to change their flow or Theological characteristics under the influence of an applied electric field. In both instances, these induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in rheological properties in the presence of a modest applied field.
Since MR fluids contain noncolloidal solid particles which are often seven to eight times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Examples of magnetorheological fluids are illustrated, for example, in U.S. Pat. No. 4,957,644 issued Sep. 18, 1990, entitled xe2x80x9cMagnetically Controllable Couplings Containing Ferrofluidsxe2x80x9d; U.S. Pat. No. 4,992,190 issued Feb. 12, 1991, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,167,850 issued Dec. 1, 1992, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,354,488 issued Oct. 11, 1994, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; and U.S. Pat. No. 5,382,373 issued Jan. 17, 1995, entitled xe2x80x9cMagnetorheological Particles Based on Alloy Particlesxe2x80x9d.
As suggested in the above patents and elsewhere, a typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
The magnetizable particles are kept in suspension by dispersing a thixotropic agent, such as fumed or precipitated silica. Silicas stabilize the MR fluid by forming a network through hydrogen bonding between silica particles. This network breaks down under shear and reforms upon cessation of shear to keep the magnetizable particles suspended while exhibiting low viscosity under shear. Precipitated silica typically has a large particle size and low surface area due to its method of formation, whereas fumed silicas are typically smaller in size with larger surface area. Fumed silicas, when used, are typically surface treated. Both precipitated silicas and treated fumed silicas, however, often exhibit poor network formation, and consequently low yield stresses in the MR fluid during operation.
MR fluids may additionally contain surfactants to prevent coagulation of the magnetizable particles. For example, the magnetizable particles may be coated with the surfactant. The surfactant is typically used in amounts less than 10% by weight relative to the weight of the silica. This typically translates to a concentration of less than 0.1% by weight of the fully formulated MR fluid. As the concentration of surfactant increases, the yield stress decreases. Yield stress is an indication of the strength of the silica network. While higher amounts of surfactant would be desirable, the amount of surfactant that may currently be used is limited due to its interference with the function of the thixotropic agent.
There is thus a need to develop a stable non-coagulating suspension having a strong network formation in the presence of high surfactant concentrations.
The present invention provides a magnetorheological fluid formulation that is a non-coagulating suspension having a high yield stress with low susceptibility to free surfactant at surfactant concentrations up to about 35% by weight relative to the weight of fumed silica therein. The fluid formulation comprises a suspension of magnetizable particles dispersed in a mixture of a liquid vehicle, a surfactant and a thixotropic agent. The thixotropic agent is an untreated fumed silica having a surface area of at least about 250 m2/g, which provides a large number of bonding sites for strong network formation.